In this May 1 photo, President Barack Obama arrives for a ceremony in the East Room of the White House. Since the president agreed to speak at Notre Dame's commencement, more than 353,000 people have signed an online petition demanding the university take back the offer.

Landing the president of the United States as commencement speaker is like grabbing the brass ring on the graduation-day carousel – usually. But for the University of Notre Dame, President Obama’s scheduled appearance Sunday as commencement speaker has touched off a gargantuan flap among Roman Catholics, not only about his policies on abortion and embryonic stem-cell research, but also about the role religion should or should not play in political life.

For a president who has taken care to embrace religious inclusivity, the vehement objections among some Catholics to his appearance at Notre Dame in South Bend, Ind., would indicate that being included may not be enough to keep certain values voters in Mr. Obama’s column.

“He claims he tries to be inclusive, but that doesn’t extend to practicing Catholics,” says Susan Fani, director of communications at The Catholic League for Religious and Civil Rights, calling the president “hostile to Catholic beliefs.” “Considering he has been in office for 100 days,” she adds, “we’re surprised at the pace that Catholic sensibilities have been offended repeatedly.”

The debate among Catholics over the president’s commencement role at the most high-profile Catholic university in the US has been heated, but it is, perhaps, not surprising. There is a well-documented political split between observant Catholics – those who attend services at least weekly – and Catholics who attend church less frequently, with the former tending to vote Republican and the latter up for grabs. In the last election, though, Obama won half the observant Catholics and more than two-thirds of less-observant worshippers.

“There’s been a lot of discussion in the community – apart from this flash point at Notre Dame – about how we engage people with different perspectives,” says the Rev. Donald Senior, president of Catholic Theological Union, a theology school in Chicago. “A lot of the Catholic community is conflicted” regarding Obama, says Father Senior, because his stance on issues like poverty and economic inequity parallels church teachings, while his support for abortion rights and embryonic stem-cell research does not.

He suggests that the church’s leaders today are grappling with how to present a public face of Catholicism in a way that resonates in a positive light rather than always appearing to be unyielding and “the church of no.”

“Maybe the church leadership is trying to say, how do we navigate this [era of hot-button issues]? It’s much more dicey now than it was 30, 40 years ago” when John Kennedy became the first Catholic president of the United States, he says.

Slippage among Catholic voters

There are signs that some Catholics, 54 percent of whom voted for Obama in November, are rethinking their views of the president. For instance, the share of Catholics who disapprove of the president’s performance is rising. Only 20 percent disapproved in February; lately that share has jumped to 45 percent, according to a poll released two weeks ago by the Pew Research Center's Forum on Religion & Public Life.

That means a majority of Catholics still approve of Obama’s performance, but support among observant Catholics – those who attend services at least once a week – is fading. Sixty-three percent of that subset is disapproving, compared with 43 percent of Catholics who go to church less often.

The White House, on Tuesday, acknowledged the controversy over the Notre Dame commencement speech, but sought to play down the extent of the opposition.

“It appears as if the vast majority of students and the majority of Catholics are supportive of the invitation the president accepted,” said Robert Gibbs, White House press secretary. “I think there is one group organizing a boycott, and as best I can understand it, there are 23 groups that have formed in support of the president's invitation.”

In a prepared statement, Notre Dame president John Jenkins said the university found Obama "an inspiring leader" who was being honored especially because he is "our first African-American president." "Racial prejudice has been a deep wound in America, and Mr. Obama has been a healer," the Rev. Mr. Jenkins said. "Of course, this does not mean we support all of his positions."

Abortion as the 'line in the sand' issue

Though Catholics in the US are split over their acceptance of abortion, the Catholic Church is definitive in its opposition. That position has always been “in the architecture of Catholic teaching,” but only in the past quarter-century has the church’s public identity been so closely linked to its abortion stance, says Richard Rosengarten, dean of the University of Chicago Divinity School. The war in Iraq and capital punishment also created opportunities for church leaders to raise their voices, but they chose abortion as the “line in the sand” issue, he says.

“One of the struggles that the bishops have today is that the political discourse is so intensified, so that any time the church is placed next to a political issue, the challenge is to figure out whether to engage and, secondly, how. It’s very, very difficult,” says Mr. Rosengarten, who is Catholic. “I think the bishops are saying, ‘We can tolerate a certain element of ambiguity about capital punishment and Iraq, but where we really have to make the nonnegotiable is [abortion].’ ”

Other beefs

The Notre Dame controversy is creating opportunities for Catholic advocacy groups to outline a number of disagreements with the president and Democratic lawmakers.

One is an antihate-crime bill, sponsored by Sens. Edward Kennedy (D) of Massachusetts and Patrick Leahy (D) of Vermont, that some Catholics say will result in a priest being prosecuted if someone commits a hate crime as a consequence of a church teaching against, for instance, homosexuality.

Another was Obama’s pledge in early March to “make scientific decisions based on facts, not ideology” regarding stem-cell research. That “was taken as special slap to religious groups,” says Richard Doerflinger, associate director of the US Bishops' Secretariat for Pro-Life Activities.

“I do think some Catholics who voted for the president thinking he wasn’t really going to take that strong a stand against the church on life issues, some of those people have reasons to be disappointed,” says Mr. Doerflinger. “We have to see what more is coming.”

Some Catholics objected, too, when they learned that certain religious iconography was removed from the backdrop during Obama’s speech in April at Georgetown University in Washington, also a Catholic university.

Kinship with evangelicals

As Catholic groups raise concerns, they are finding an unlikely ally: Protestant evangelicals.

“The core values of the Catholic church are our core values,” says evangelical leader Lou Engle, who is considering showing up at Notre Dame on Sunday in support of Catholic protesters. “It’s amazing how a battle can bring together strange bedfellows.”

Whether Catholics will splinter further over politics remains to be seen. But Democrats risk losing at least some of the gains of the last election.

The Catholic League’s Ms. Fani says because Obama’s track record with Catholic leaders has started off on such shaky footing, “they’re not going along with the Obama administration.”